In telephone systems, trunk circuits serve as an interface between communication or, more particularly, transmission lines connected to a distant switching office and the switching network of a local telephone switching office. Historically, the receiving portion of a trunk circuit includes a battery circuit to supply power to the transmission line and a detector circuit to detect the high and low impedance levels of the line. These high and low impedance levels are frequently called the on-hook and off-hook states of the line, respectively, and are often used for supervisory and routing signaling on the line. The distant office may request use of the line by causing the line to assume the low impedance level or off-hook state. When the line is in the off-hook state, the battery circuit causes sufficient current flow in the line to operate the detector circuit at the switching network. After the initial off-hook state has been detected, the detector may be utilized for the detection of routing and further supervisory signals on the line.
Generally, standard battery circuits are designed for a maximum length line. For short lines, the standard battery circuit will tend to deliver an excessive amount of current. Elimination of high current consumption in short lines is desirable not only to protect the line apparatus but also to allow a reduction in power dissipation requirements of battery circuits and to reduce the generation of heat and the overall current consumption of the switching office. Battery circuits which provide a constant current to the line independent of the line length are well-known in the prior art.
Since the impedance of the line varies directly with the length of the line, a constant current battery circuit will supply a wide range of voltages. Operating within this wide range of voltages, detector circuits and the like commonly used in the current art have been able to detect state transitions of both long and short lines where the difference between the minimum on-hook (high) impedance of a short line and the maximum off-hook (low) impedance of a long line is large, such as, e.g., 24,000 ohms. This situation is commonly found in lines where the on-hook state presents an open circuit condition. However, in continuously terminated lines where the impedance difference may be minimal, such as, e.g., 400 ohms, the current art detector circuits cannot detect the difference in state between an on-hook short line and an off-hook long line.